720 A'AAS.^S CITY REVIEW OF SCIENCE. 



have been developed from pre-existing forms by physical means, that any real 

 progress was made in the solution of this problem. As many of these changes 

 must have occurred in the remote past, it is necessary to know the history of 

 their geologic succession. Palaeontology, therefore, affords us the primary basis 

 upon which both advocates and opponents of this theory must rest their opinions 

 and formulate their beliefs. Evidence bearing directly upon the past succession 

 of several groups of the Mammalia has of late years been accumulating to such 

 an extent as to render it now possible to construct their phylogeny and discern 

 with a comparative degree of certainty the exact lines through which they have 

 descended to their present condition. 



For this purpose I have selected as the subject of this article a member of an 

 order known to naturalists as the Perissodactyla, a name derived from two Greek 

 words meaning odd-toed. In defining the systematic position of this animal it 

 will be necessary to discuss the classification of, first, the order to which it 

 belongs, and second, the various sub-divisions included within the order itself. 

 In the pursuit of this course the use of technicalities is unavoidable, but these I 

 will endeavor to make as clear as the circumstances will permit. 



As considerable has already been written upon the genealogy of the horse by 

 others, it might appear imprudent for me to say more. But unfortunately the 

 nomenclature that has been used renders it quite impracticable for the palaeontol- 

 ogist to ascertain to what forms reference is made. The genera Eohippus, Oro- 

 hippus, Miohippus and Fliohippus, have not, in the author's estimation, been dis- 

 tinguished from genera previously described and figured by other authors; hence 

 my reasons for adopting names more in accordance with the prevailing nomencla- 

 ture of the science. 



The Perissodactyla may be defined as mammalian animals having both pairs 

 of limbs fully developed and adapted for walking or running. Toes with terminal 

 phalanges encased in strong corneous sheaths developed as hoofs. But as 

 these characters apply to two other orders also, viz: the " cloven-hoofed " or 

 Artiodactyla, and the " short- footed " or Amblypoda, it will be necessary to con- 

 sider the characters that establish the differences between these natural groups. It 

 must be observed that the anatomical features that separate these orders are to be, 

 found mainly in the structure of the?r hind limbs. A comparison with the Artio- 

 dactyla shows that the hind feet always possess an odd number of toes, while in 

 that order the number is permanently even, as far at least as our present knowl- 

 edge extends. The third digit of each foot is symmetrical in itself, which is not 

 the case in the Artiodactyla. An important bone of the " hock " joint which cor- 

 responds to the ankle-bone of man, and to which the name astragalus has been 

 given, presents either a single articular face to the navicular bone below or has, 

 more commonly, a small facet in addition for articulation with the cuboid bone. 

 The femur, or thigh-bone, possesses a strong third trochanter for the insertion of 

 the gluteus niaxinius muscle, a process not found in the corresponding bone of the 

 " split-hoofed " division. The astragalus in the ^r/ZiC'^/arfj'/^ has its inferior sur- 

 face divided into two subsequal facets, which are very convex antero-posteriorly 



